Ocean Carbon Sequestration
The Deep in Deep Blue Carbon
Ocean based negative carbon has another important advantage over land-based techniques: A VERY easy way to sequester the carbon safely for a LONG time.
The oceans can hold ALOT of CO2. It is ALREADY the case that the worlds oceans have absorbed 30% of all human emissions so far. Indeed if we stopped emitting CO2 and waited a few thousand years, the worlds oceans would reduce the CO2 in the atmosphere from the current 410 ppm, to under 350 ppm (a good target amount). See IPCC Ocean Storage). We just can't wait that long.
The problem is that only a tiny 100m sliver at the top of the ocean (called the mixed layer) readily interacts with the atmosphere. The ocean is DEEP (See ocean depth map. Mean depth > 3km) , so most (97%) of it is not helping.
However if we drop biomass into the deep ocean (well beyond he mixed layer, say 1km or more) it will decompose into C02 and dissolve in the water, but that CO2 is at the BOTTOM of the ocean not the top. and mostly just sits there.
There are VERY slow currents (see Thermohaline circulation) but they take many hundreds of years to cycle, and as this (long) time goes by the CO2 from the decayed biomass slowly diffuses into more of the ocean. By the time the bottom water reaches the surface, the CO2 in the water has diffused through enough water and would rather STAY in the water than to leak into the atmosphere.
This gives us a very simple system for sequestering carbon. Grow seaweed in the open ocean and let some of its biomass sink to the bottom. Most of the carbon associated with that biomass will never reenter the atmosphere (it stays in the water). This is about as simple as you could imagine CO2 sequestration getting.
Certainly there are legitimate concerns about the effects of dumping all of our CO2 into our oceans will have. Tempering this concern is the fact that most life resides in the top 100m that has already had CO2 dumped into it. We can't easily fix that issue and the effects of deep CO2 are likely to be small by comparison. We have to pick the least bad option.